1. Field of the Invention
The present invention relates to a process for preparing a carboxylic ester. In particular, the present invention relates to a catalytic distillation process utilizing acidic catalysts in solid phase for the preparation of carboxylic esters. This catalytic distillation process is especially suitable for the preparation of carboxylic ester having a relative volatility with the reactant: carboxylic acid, close to 1.
2. Background of the Invention
U.S. Pat. No. 5,719,311, the content thereof is incorporated herein by reference, discloses a fixed bed catalytic process for the esterification of carboxylic acids and alcohol into carboxylic esters. In the process disclosed in the ""311 patent, the fixed bed reactor contains acidic catalysts that are present in solid phase, and the reaction condition is controlled such that (i) the reactants and the products co-exist in a gas-liquid two-phase equilibrium in the fixed bed reactor and that (ii) at least one component of the reactants is present in one phase and at least one component of the products is present in another phase. Very high reaction yield and selectivity, typically better than 90%, sometimes exceeding 99%, were observed with the process disclosed in the ""311 patent for preparing the esters of methanol/propionic acid, methanol/methacrylic acid, isobutanol/hexahydrophthalic anhydride, and isooctyl alcohol/phthalic anhydride. However, when the carboxylic acid is acetic acid, which has high miscibility with many alcohols, the reaction yield from the process of the ""311 patent will not be the same high level as with other carboxylic acids, and conventional processes need to be used to produce esters of acetic acid in an economic manner. Lowered reaction yield causes unsatisfactory amounts of alcohol to remain in the production, thus further adversely affecting the economic potential of the process, especially with regard to post-esterification purification cost.
To solve the problem encountered with the preparation of acetic esters, U.S. Pat. No. 5,998,659 proposes a catalytic process for the preparation of acetic esters. According to the proposed process, a fixed bed catalytic esterification and a catalytic distillation are utilized. This catalytic process is able to achieve excellent esterification yield of acetic acid and provide post esterification with substantially reduced height of the distillation column. However, this process is not suitable for the preparation of carboxylic esters that has a relative volatility with the reactant, carboxylic acid, close to 1, because carboxylic esters with high boiling point are not suitable for discharge from the top of the catalytic distillation column. Collection of esterification products from the top of the catalytic distillation column necessitates a large amount of refluxed water, making the process impractical and uneconomical.
It is therefore an object of the invention to provide an improved process for preparing carboxylic esters having high boiling point from carboxylic acids and alcohols.
It is another object of the invention to provide a process for preparing carboxylic esters, which is able to achieve excellent esterification yield, sometimes exceeding 99.8% and thus reduce the post-esterification separation and production costs.
To attain the objects mentioned above, the process of the invention utilizes a catalytic distillation column containing acidic catalyst in solid phase. In accordance with the present invention, the esterification reaction can be accomplished in this catalytic distillation column, or the catalytic distillation column can be connected to the downstream of an esterification reactor. The catalytic distillation column is so designed that when no esterification pre-reactor is used, the alcohol is fed from the bottom portion thereof, and the carboxylic acid is fed from the top portion. When a pre-reactor is used, the alcohol is fed from the bottom portion of the distillation column, and the reaction product from the pre-reactor is fed from the top portion thereof. In other words, the esterification and extraction of the esterification product are conducted in a counter-flow manner. Note that the residence time of the carboxylic acids in the catalytic distillation column must be sufficient to accomplish high reactant conversion and to produce carboxylic esters and water. Also, note that according to the process of the invention, the alcohol and water are continuously removed from the top of the catalytic distillation column, and the carboxylic esters are continuously removed from the bottom.
By using the catalytic distillation process, the distillate from the bottom contains more than 99.8% of the carboxylic esters. Because of the high esterification yield, the post-esterification purification can be simplified and the amount of energy consumption can also be greatly reduced. Also, a mixed solution discharged from the top of the catalytic distillation column is then separated by a separator, for example a decanter, into water phase and organic phase. The organic phase, comprising alcohols, carboxylic esters and carboxylic acids, is directly refluxed into the catalytic distillation column and the water phase is stripped to recycle residual organic composition. By using the catalytic process of the invention, the amount of energy consumption can be greatly reduced to ⅔ of a conventional catalytic process, and in practical application 1-2 purification distillation columns can be saved.
The carboxylic acids of the esterification reactants, which may be suitable for use in this invention, are straight-chain, branched, and aromatic carboxylic acids having a carbon number from 2 to 10. An anhydride of the above-mentioned carboxylic acids is also suitable for the purpose of the invention. The catalytic process of the invention is especially suitable for the esterificatin of propionic acid and acetic acids which are the esterification reactants of commercially available propionic esters and acetic esters.
Other esterification reactants, alcohols can be straight-chain, branched and aromatic alcohols having a carbon number from 1-12. Note that according to an aspect of the invention, the carboxylic esters and alcohols must be so selected that the resulting carboxylic esters have the highest boiling point in the whole reaction system and have a boiling point higher than 100xc2x0 C.
The catalytic distillation column utilized according to the invention can be one which includes several sections disposed vertically, each section respectively filled with packing material or solid acid catalysts. For example, a catalytic distillation column, including, from the bottom to the top, stripping section, reaction section, extractive distillation section, and rectification section, is preferred. The stripping section and rectification section can be filled with packing materials, such as PRO-PAK packing material available from Sulzer. The reaction section and the extractive distillation section are filled with solid acidic catalysts. The solid acidic catalysts suitable for use in this invention include but are not limited to oxide, zeolite, cation-exchange resins or a mixture thereof. Among them, SO42xe2x88x92/ZrO2 oxide, or HZSM-5 zeolite, is preferred.
As mentioned in the summary section, the catalytic distillation column can be connected in series with a esterification pre-reactor. Preferably, the pre-reactor is a fixed bed reactor inside which is packed with solid acid catalyst. Also, preferably, the fixed bed reactor is controlled under a reaction condition that reactants and reaction products co-exist in a gas-liquid two-phase equilibrium and that at least one component of the reactants is present in one phase and at least one component of the reaction products is present in another phase. For example, in the preparation of N-butyl propionate, in the fixed bed reactor, the propionic acid is in liquid phase and is charged into the upper portion of the fixed bed reactor together with alcohol in gas phase, while the resulting N-butyl propionate and water are discharged from the bottom portion thereof in gas phase.
Without any intent to limit the scope of this invention, the present invention will be hereinafter explained in more detail with reference to the examples. Note that in the following examples, the reaction yield of the carboxylic ester is defined as below.   Yield  =            moles      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      carboxylic      ⁢              xe2x80x83            ⁢      ester      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      product              moles      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      alcohol      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      feed      